.slider {
  -webkit-appearance: none;
  width: 100%;
  height: 15px;
  border-radius: 5px;  
  background: var(--blau);
  outline: none;
  opacity: 1;
  -webkit-transition: .2s;
  transition: opacity .2s;
}

.slider::-webkit-slider-thumb {
  -webkit-appearance: none;
  appearance: none;
  width: 20px;
  height: 20px;
  border-radius: 50%; 
  background: #fff;
    border:2px solid var(--blau);
  cursor: pointer;
}

.slider::-moz-range-thumb {
  width: 20px;
  height: 20px;
  border-radius: 50%;
  background: #fff;
  cursor: pointer;
    border:2px solid var(--blau);
}